212 Dr Searle, Experiments illustrating 
with that found by (12) from f and f,. If we equate these two 
values of «—1, we obtain 
21 PA) ae cence (16). 
Fig. 3. 
Equation (13) can also be obtained as follows:—Since S coincides — 
with its own image, the ray SGH (Fig. 3) must be normal to the 
surface BK. Hence the emergent ray HZ is directed from O, the 
centre of curvature of BK, and thus O and S are conjugate — 
i 
poits. When the lens is thin, we may put MO=b and MS=p. — 
Then 
Up —1/b=1/f 
which agrees with (13). The method of determining the radius b 
by making an object S coincide with its image by reflexion at BK ~ 
is due to C. V. Boys. 
If BK be so strongly concave that 1/f+ 1/b is negative, p will 
be negative and then it will be impossible to make a real object 
coincide with its own image, and Boys’s method cannot be 
applied. Since, however, the surface BX is concave, the radius b 
can be found directly by making an object on the B-side of the 
lens coincide with its image by reflexion at BK. If AK be 
smeared with vaseline, no image will be formed by reflexion at 
AK and thus confusion will be avoided. 
§ 9. Haperimental details. A spectacle lens (price 9d.) is 
suitable for the experiment; its primary focal length should be 
considerable—say, a metre,—so that its thickness may be neg- 
lected without much loss of accuracy. The primary focal length f 
is found by aid of a very distant object (500 metres) or of a good 
plane mirror, or by measuring the distances of an object and its 
image from the lens. An ordinary optical bench will not be long 
enough to allow the focal length to be conveniently found by the 
minimum distance method. 
The secondary focal length /, is found on an optical bench 
(Fig. 4). One of the sliding carriages of the bench carries a tube 
